Collaborative work continues into the relationship of genetic and nongenetic factors in spontaneous mouse tumorigenesis. Interest is in the different lines of naturally occurring mammary tumor virus (MTV), and in the genetically variable inbred mouse strains. Interest also is in the manner of transmission of the various MTV's, i.e. whether by female or equally well by either parent. Finally, strains of mice that develop hormone responsive mammary gland tumors and hyperplastic alveolar nodules from which malignant mammary tumors arise are also of interest for histogenesis studies.